1. Field of the Invention
The present invention relates to a substrate heating device, which heats a substrate such as a semiconductor substrate, a liquid crystal substrate or the like. More specifically, the present invention relates to a substrate heating device which includes a ceramic base plate and resistance heating elements buried in the ceramic base plate.
2. Description of the Related Art
A ceramic heater which has a ceramic base plate and resistance heating elements buried in the ceramic base plate is widely used as a substrate heating device in a semiconductor manufacturing process or the like. The ceramic heater used in a semiconductor manufacturing process or the like is available over a wide range of temperatures for applications. Uniform heat on the heating surface of the heater is desired for increasing yield of semiconductor products.
In many ceramic heaters, a single resistance heating element, which is provided by forming a single continuous linear resistance heating element into a coil shape, is buried in the ceramic base plate. Recently, a multi-zone heater having independent resistance heating elements buried in respective zones into which the heating surface is divided has been adopted. A heating value may be specified for each zone in the multi-zone heater, thereby providing improved uniformity of heater surface temperature. A multi-zone heater with resistance heating element buried in respective nine zones of the ceramic base plate is disclosed in Japanese Patent Application Laid-Open No. 2001-52843, FIG. 1.
In the conventional multi-zone heater, terminals are connected to the respective resistance heating elements. Lead wires are connected to the respective terminals and are connected to a power supply line. In the conventional ceramic heater, the lead wires are also buried in the ceramic base plate. Therefore, the lead wires are wired in the ceramic base plate and connected to the power supply line at the central of the underside of the ceramic base plate.
Meanwhile, in this case, since the buried lead wires themselves may become defects of the ceramic base plate, the thinnest available wire must be used as lead wires to prevent generating stress. In addition, in the manufacturing process for the ceramic base plate, since a ceramic compact in which lead wires are buried is sintered, the lead wires must be made of a sintering temperature-proof refractory metal.
Therefore, conventionally, a refractory metal having a high volume resistivity has been used for lead wires as with a resistance heating element material. As a result, the lead wires generate heat as with the resistance heating element. When extending the respective lead wires to the central of the ceramic base plate, the lead wires must always be arranged to pass through other zones. Therefore, it is difficult to precisely control the temperatures of the respective zones due to influence of heat generation of the lead wires.